Add additional fields from the SNIRF data format documentation

[dsleiter]

Based on the SNIRF data format documentation here: https://docs.google.com/document/d/1kLQnFNSXsCAcUNcCPtyTerSwatCOnLcX4fAaZsidCbI/edit

The following are characteristics of the device / measurement schema, and initially we plan to add as attributes to NIRSDevice:

• nirs_mode (continuous wave, frequency, time domain, etc) [required]
• frequency (for frequency domain) [optional]
• time_delay (for time domain) [optional]
• time_delay_width (for time domain) [optional]
• correlation_time_delay (for DCS) [optional]
• correlation_time_delay_width (for DCS) [optional]
• comment: str (for anything else useful, such as additional parameter values) [optional]

The following are chararacteristics of individual channels, so we plan to add as columns to NIRSChannelsTable:

• change wavelength -> source_wavelength [required]
• emission_wavelength [optional]
• source_power [optional]
• detector_gain [optional]

Note: this issue is intended to just create a place for this data into the extension, and we should expect some reorganization in the future, especially after getting feedback from the NWB team.

[dsleiter]

The original hdmf PR has been replaced by this one: https://github.com/hdmf-dev/hdmf/pull/536, which has been merged-in. We're just waiting on the next releas for hdmf (and maybe pynwb)